Twenty-two owl monkeys were inoculated intracerebrally, subcutaneously and intravenously with JC, BK or SV-40 strains of papovaviruses. Two adult owl monkeys inoculated with JC virus, a human polyomavirus, developed brain tumors at 16 and 25 months postinoculaton, respectively. A Grade III to IV astrocytoma (resembling a human glioblastoma multifore) was found in the left cerebral hemisphere and brainstem of one owl monkey. The second owl monkey developed a malignant tumor in the left cerebral hemisphere containing both glial and neuronal cell types. Impression smears prepared from unfixed tissue of this tumor showed cells which contained polyomabirus T antigen. Virion antigens were not detected. Tumor cells cultured in vitro also containnd T antigen but were negative for virion antigen. Infectious virus was not isolated from extracts of this tumor. Additional animals have been inoculated with JC virus to confirm these findings.